The cracking of hydrocarbons brought about by heating a feedstock material in a furnace has long been used to produce a variety of useful products. For example, ethylene, which is among the more important products in the chemical industry, can be produced by the pyrolysis of feedstocks ranging from light paraffins, such as ethane and propane, to heavier fractions such as naptha. Typically, the lighter feedstocks produce higher ethylene yields (50-55% for ethane compared to 25-30% for naphtha); however, the cost of the feedstock is more likely to determine which is used. Since the pricing of feedstocks that may be used to produce a desired product is very volatile, it is preferable to design a pyrolysis plant to use a variety of feedstocks, enabling the most cost effective feedstock to be used in response to changing market prices.
Energy consumption is another cost factor impacting the pyrolytic production of chemical products from various feedstocks. Over the last two decades, there have been significant improvements in the efficiency of the pyrolysis process that have reduced the cost of production. In the typical or conventional pyrolysis plant, a feedstock passes through a plurality of heat exchanger tubes where it is heated externally to a pyrolysis temperature by the combustion products of fuel oil or natural gas and air. One of the more important steps taken to minimize production costs has been the reduction of the residence time for a feedstock in the heat exchanger tubes of a pyrolysis furnace. Reduction of the residence time increases the yield of the desired product while reducing the production of heavier by-products that tend to foul the pyrolysis tube walls. Heat transfer rates can be increased by using smaller diameter, shorter tubes for the pyrolysis heat exchanger and residence times in the range of 50-100 milliseconds can thus be obtained (compared to 250-1000 milliseconds in earlier design conventional furnaces). In the production of ethylene, this provides a 5-10% increase in the yield when ethane is used as a feedstock, and a 20-30% increase in the yield for heavier feedstocks. A furnace having this range of relatively short residence times is sometimes referred to as a "millisecond furnace." Even greater increases in yield and reduction in coking are potentially available by further shortening the residence times; however, the design of millisecond furnaces of this type does not readily permit further reduction of the residence time.
An alternative approach to heating a petroleum feedstock to a cracking or pyrolysis temperature is disclosed in U.S. Pat. No. 4,136,015. The process taught in this patent involves the injection of an atomized petroleum feedstock into a stream of hot combustion products formed by the combustion of a fuel (H.sub.2 or CH.sub.4) mixed with oxygen. The fuel and oxygen mix with steam in a mixing zone of the furnace and subsequently burn in a combustion zone, producing hot products of combustion that travel at subsonic velocity into a constricting throat of the furnace. The liquid feedstock is injected into the stream of hot combustion products just upstream of the constricting throat. In the throat, the atomized liquid feedstock is mixed with and vaporized by the hot combustion products, causing an initial cracking or pyrolysis of the feedstock to occur. The mixture passes into an acceleration region where its velocity increases to a supersonic level (between Mar. 1 and 2). A cross-sectional expansion of the pyrolysis zone and back pressure developed downstream produce a shock wave that increases the temperature of the mixture, effecting a final thermal cracking of the feedstock. Downstream of the shock wave, the velocity of the stream slows to a subsonic level, but the cracking process continues until the stream enters a quenching zone in which it is cooled by water injection. Finally, the quenched stream passes into a heat exchanger that recovers usable energy. This reference teaches that operation of the furnace in the supersonic mode increases the production of ethylene by about 7% and substantially reduces the accumulation of burnt carbon (or coking).
In Russian Patent No. SU 392723A, a related method is disclosed for producing acetylene and ethylene via the oxidizing pyrolysis of methane. The oxidizing pyrolysis occurs in a supersonic outflow of a heated mixture of methane and oxygen from a supersonic nozzle. The reference teaches that a shock wave created in the supersonic flow raises the yield of the end products. In this method, methane and oxygen are separately preheated in heat exchangers to 800.degree. C. and are then fed into a preliminary mixing chamber. Since the induction period is short, the mixture does not immediately ignite. Instead, the mixture cools and gathers velocity as it expands through a nozzle, achieving a supersonic speed (1500 meters/second). Upon exiting the nozzle, the supersonic stream of mixed gases impacts against a barrier, which raises the temperature sharply to the ignition temperature, causing a violent combustion of the mixture to occur. Methane molecules in a high-temperature layer of the mixture are pyrolized to ethylene and acetylene, but since the methane only resides in a high-temperature zone of the furnace for a very brief period, secondary reactions are of less importance. Water is injected into the mixture in which the methane is undergoing pyrolysis to quench the reaction.
In both of the above-referenced patents, the energy required to raise the feedstock temperatures to initiate pyrolysis is provided by combustion of feedstock or carrier fluid in an oxygen-rich environment. In the process described in U.S. Pat. No. 4,136,015 the feedstock is partially pyrolyzed by direct exposure to combustion products at temperatures as high as 2200.degree. C., and partially pyrolyzed in the shock-heated zone downstream. As a result, uncontrolled pyrolysis of the feedstock occurs prior to heating caused by the shock wave. The temperature of the products of combustion and feedstock is thus likely to remain sufficiently high so that some coking in the furnace occurs upstream of the shock wave. In Russian Patent No. SU 392723A, the shock wave initiates combustion, again producing very high temperatures at which the feedstock is pyrolized. Accordingly, it will be apparent that merely providing a short residence time downstream of the shock wave is not sufficient; the feedstock should remain below the pyrolysis temperature until the pyrolysis reaction is initiated in a pyrolysis region. In addition, the residence time of the feedstock in the pyrolysis region of the furnace should be as brief as possible and precisely controllable to enable different feedstocks to be pyrolized, with maximum yield of the desired product.